Method of coating



' ,Aprii 17,` 1343. L. V@QLD 2,373,323

METHOD oF co'ATING Filed June g8. 1942 /NVENTOR .L GQLD ar TTOPNEV l 'Patented pr. 17,1945

Louis Gold, Brooklyn, N. Y., assigner to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application June 18, 1942, Serial No. 447,548

('Cl. 11h-35) 4 Claims.

'I'his invention relates to a method of coating,

and more particularly to a method of coating a y surface of a solid body with metal.

In various recent developments in branches of the electrical arts, it has become important to be able to provide accurately formed bodies of electrically insulating material, e. g. cut and polished slabs of crystalline silica or quartz or other crystallinetnon-conducting material, with a closely adherent deposit of metal of uniform thickness on one or more of the surfaces of such a slab, the metal coating .so made 4being of uniform and closely predeterminable thickness and of-uniform density.

An object ofA the present invention is to provide a method of forming a predeterminably thick coating of uniform thickness and density upon a surface of a.solid body, with reliably constant exposed upper faces.

rial, e. g. ceramic material, heat resistant glass or the like, although the tray may also be of suitable metal provided on the under surface of its bottom with electrically insulating supports or an electrically insulating coating, serves to support the quartz slabs II to be coated with silver on their There may be a single slab II large enough to cover substantially all or a results and simplicity of means and of operation.

With the above and othervobjects in view, the invention may be embodied in a method of coat- .ing the surface of a solid body with metal and major portion of the floor of the tray; or, as illustrated, a plurality of slabs I I to be simultaneously coated' may be disposed on the tray floor preferably to cover substantially all of the floor, although fewer than this'may also be treatedat one time. For the purposes of the present invention, the upper surface o'f a single large slab occupying substantially all of the tray oor, or

the combined 'upper surfaces of a plurality of smaller slabs simultaneously extending over substantially all of the floor surface, will be spoken of as the surface to be coated or treated regardless of whether it is actually one continuous surface or a mosaic of surfaces of a plurality of` comprising -steps of distributing an elongated supply of evaporable metal adjacent to and parallel with a surface to be coated in such manner that the supply of evaporable metal issubstantially constant in amount per unit length thereof except that the amount of supply per unit length is materially increased wherever the supply approaches'an edge of the surface, and then' enclosing the supply and the surface tovbe coated together in an evacuated space, and heating the supply to evaporate the same to'coatthe surface.

Other objects and features 'of the invention will f appear from the following detailed descriptionof one embodiment thereof taken in connection with the accompanying drawing in which the same reference numerals are applied to identical parts in the several gures, and in which Fig. 1 is a diagrammatic view in side elevation with parts broken away, of an illustrative apparatus constructed to carry out the method in accordance with the invention;

Fig. 2 is an enlarged detached broken view in side elevation of a portion thereof; Fig. 3 is a sectional View on the line 3-3 of Fig. 1; andI Fig. 4 is a modification of the showing of Fig. 2.

The apparatus herein disclosed to illustrate the -method of the invention has for its purpose to form a lm of metallic silver of uniform'thickness and density upon one or a4 simultaneously coated plurality of slabsv end a rectangular of crystallinel quartz. To this' tray I0, preferably of high heat juxtaposedslabs. Similarly, when an end edge or the two end edges of the surface `to be treated or coated are spoken of, such end edge isto be taken to mean the edge of' a single large slab nearest to one end of the tray, or the combined colinear edges 'of a plurality of slabs nearest one end of the tray. Thus, as illustrated in the drawing, the surface to be coated or treated is the mosaic surface of all of the slabs II in the tray I0; and the two end edgesv of this surface are respectivelythe edges.'l I2 and I2 taken together and the edges I3 and I3 taken together.

AA horizontally elongated housing I4, having an evacuable chamber I5 therein, is rigidly supportedy in any` suitable manner (not shown).

Preferably, as illustrated, this housing and chamber are cylindrical, although they may have any suitable form. At the front or vleft end of the chamber is an opening closed by a removable and hermetically sealable door or plug I6. Rigidly secured in the inner face of the door I6 is a pair'of parallel, horizontally extending, rigidA bars or rods II.= These rods are preferably of elecltrically conductive and rigid metal, steel, hard v brass, bronze or the like; and the door vor plug the rods I1 mustI be electrically insulated from the plug or door where they are mounted in it..y

resistant and electrically non-conducting mate- 55 I6 is preferably o f insulating material, e. g. porcelain or other ceramic material.- The door I6 may e also be of suitable metal however, in which case The bars Il are spaced apart a suitable distance to allow the tray I0 to be placed and safely supported thereon.v Twin binding posts' I8 of electerial.

`22 mounted on the same bar Il.

- the surface to be coated.

respectively on the bars I1 near their left or front ends, and are electrically insulated from the bars by sleeves I9 of ceramic or other suitable ma- Twin metal blocks 20 are vmounted respectively on the bars I1 and in electrical contact therewith near the right or rear-ends thereof. The blocks 20 are so spaced from the posts I8as to receive the tray I easily therebetween. On

each block is a vertical, laterally resilient upl0 right support 2l, e. g. a vertical helix of stainless steel or other suitable electrically conductive metal. On the top of each support 2| is secured a metal binding post 22. The horizontally disposed wire receivlng'bores of the four clamping 15 y heads of the binding posts I8, I8, 22 and 22 are all substantially equidistant from the plane of the fioor of the tray I0 when the tray is in place on the bars I1..

A wire helix 23 is clamped in and extends between each binding post I8 and the binding post Thus, in the illustrated apparatus, there are two helices 23 supported parallel to each other and above and 1trically conductive metal. are rigidly mounted and from the side edges of the surface, the surface is satisfactorily coated over its entire extent except for a transverse strip adjacent 'each end edge of the surface where the coating thins progressively toward eachend edge. The width of this strip varies somewhat with varying proportions and dispositions of the parts,but seems to be always of materially important extent and represents an annoying and even costly waste of both s'ilver'and-crystal slab. It may, of course, be obviated by making the length of the helices 23 and of the silver wire therein enough greater than the distance between the end edges of the surfaces to be coated to ensure that the insuiiiparallel to the floor of the tray I0 on the bars I1. 'g5

All the. dimensions of the apparatus as thus far described will Vary with the size of the surface to be coated and must be .determined .ac-

' cordingly. In one particular instance of actual practise, the surface to be coated was approxi- 3g cient coating effect is wasted beyond the end edges. However, this is not only a costly waste of silver, but also tends to build up encrustations of silver on the bars I1 and even perhaps to solder these bars to the contacts 24.

In accordance with the present invention, it has been discovered that a uniform coating from end edgel to lend edge of the surface to be coated can" be effected, with negligible waste of silver beyond the end edges, if the silver supply, substantially constant elsewhere per unit lengthI be materially increased over the portions of the surface to be coated near each of its end edges, apparently over a distance comparable to the width ofthe transverse strips insufficiently coated by a'coinpletely uniform supply. In accordance with `the invention therefor, the silver supply wire 38 is made about equal in length to the distance aboutv thirteen and one-half inches long between with about twelve and one-half coils to the inch and an internal diameter of its coils of about one-sixteenth of an inch.

Resilient contact members 24 carried on binding posts 25 suitably mounted in and passing through the wall of. the chamber I5, make resil- 45 ient electrical contact with the bars I'I. The clamping heads 25 of the posts 25 outside the housing I4 are connected to a. source of electrical power (not shown) and also to suitable voltage and amperage indicating devices 21 and 3 2 re- 50 spectively. An exhaust 4pipe 28 communicating vthrough the wall of the housing I4 with the chamber I5, is connectedto some suitable evacuating device (not shown).

Before the helices 23 are secured in position, as 55 shown, in the binding posts I8 and 22, there is, threaded into the cylindrical axial space of each helix an elongated body of silver to be the supply of silver to'be evaporated and deposited upon The obviousway to do this is to cut apiece 30 of silver vwire equal in length to the distance between the end edges of the surfaceV to be coated and then to insert such a piece of wire into each of the helices 23.

The chamber I5 is then evacuated, current is supplied to the helices 23 to heat them, through the contacts 24, and the silver wire heated by the helices is evaporated or sublimed and deposits a coating of silver below. .y If the procedure be undertaken in this manner', it is found that, witlrthe helices 23 suitably dis' posed over the surface to b'e coated as to distance on the surface in the tray I0 70` `from the surface and vdistance from each other between the end edges of the surfaceto be'coated but it is increased in mass near each end, either by winding a detached extra piece of' the silver wire over a suitable length of the main silver wire vas shown at |30 at the left end of Fig. 2, or by turning back and similarly coiling an undetached extra length as shown at 230 at the right in Fig. 2. Similarly, a bit of silver foil might be coiled on as indicated at 330 at the left in Fig. 4, or a sleeve of silver might be slipped on as shown at 430 at the right in Fig'. 4. i

The amount and distribution of the silver, thus added toeach end of the supply will vary from case to case; and the variables in volved are too many for any practicable formula predetermining this to be given. However, in the particular case for which dimensions are given above, `it was found that a coating from end edge to end edge of the surface to be coated and without materially important waste beyond these edges could be obtained by taking two extra pieces of the same silver wire as that used for the main supply wire 30, -and 'winding one on each end of the main wire overadistance-of about two inches,

`each of these pieces being about four and onehalf inches long before Winding. 'I'he silver wire used was substantially pure silver and about 0.01 inch in diameter'.

In carrying on the coating operation, certain conditions of operation arefound to be materially important. For instance, in the particular case already referred to, the following cycle of steps is preferred. 'The apparatus is arranged and prepared as already described and, then is evacuated through the tube 28 until the pressure inside is of the order of 0.00001 mm. of mercury. Current is then applied tothe contacts 24 and thus to the bars I 1. .This current flows from .fone contact 24', through the corresponding bar I'I, block 20, resilient support 2l, clamp 22, and helix 2 3, to one post I8. The tyvo posts I8 are' connected by a suitable conductor 29. The cur rent flows through. this to the other post I8 and thence back via the other helix 23, clamp 22,

support'2l, block 2li and bar l1 to the other contact 24.

The current is so regulated that it increases gradually over a period of about five seconds .to about live amperes and is held at this value for two minutes. In the next ten seconds it is brought up gradually to ten amperes.

At the' beginning ofv these periods :of current iiow, the current flows through each molybdenum helix 23 and also through the silver wire 30-I3Il supported in the helix. The silver wire is not inserted into the binding posts; nevertheless it in effect short circuits the successive coils of the molybdenum helix and carries most of the current at rst. The silver wire is thus overheated and fuses apart between'coils of the helix. At each such breakage of thev silver circuit an iny crease of voltage is registered on the voltage registering device 21. The ten ampere value of the current is maintained until no further voltage increase is registered, showing -that no helix coils remain shorted by the silver.

The dimensions of the helix 23, especially the spacing between coils or pitch of the helix, but

also the diameter of the molybdenum wire, are important. Thesemust be such that when all the silver short circuits are fused and broken, the amount of molten silver associated with each helix coil will not -be greater than the surface tension of the molten silver can retain attached and adherent to the coil, so" that no grossdroplets o f molten silver will fall from the coil to coat the surface below irregularly and excessively. At the same time the helix coils must be as widely spaced as 'may be to minimize their interference with the evaporation or sublimation of the silver as vapor down .upon the surface in a uniform manner. For the distance of the helices above the surface and from each other and `for a 0.01 inch silver wire, the lhelix of molybdenum .0.02 inch wire, about twelve and one-half coils to the inch, and with an internal helixdiameter of about 0.065'inch, is found to be satisfactory. If a greater or less volume of silver per unit length of helix is tobe evaporated, the helix dimensions will be modified accordingly and also the amperages and lengths of time of the successive steps of the process.

The silver short circuits -havingv thus al1 been broken, the amperage is reduced abruptly to slabs by silver evaporated from a supporting helix or helices of molybdenum, .the invention 'I is not so limited; other metals, e. g.- aluminum,

zinc, gold, tin, etc. may be used as the v coating material in suitably modified apparatus and by the same general procedure, to coat other substances than quartz, e.g. high silica glass, mica,

ceramic objects, slabs of almost any metal if desired, and other' lanalogous surfaces. molybdenum is the at present preferred material for the helices 23, other metals,e.`g. tungsten, may also be used. The vpreferred material for the helices will necessarily depend upon4 the nature of the coating metal. Preferably the material of the h'elix should not be materially affected or alloyed by the molten coating material, to enable re-use ofvany' given helix as often as possible. Other modifications and variations'v of the particular illustrative embodiment above described within the spirit and scope of the invention as pointed out yin the appended claims will vreadily occur to those skilled in the art. I

What is claimed is:

1. The method of coating-'a surface having an v I end edge with a coating which is uniform to the said edge and-Which method comprises steps of positioning over and generally parallel to the about ve amperes and held at that value for about ve minutes to allow the silver to form itself into probably pastily soft, not fluidly molten,

bodies on thehelix coils. This is not a known fact, for observation of what happens in the chamber I5 is not practicable. At any rate, after the last jump in voltage value indicating the f surface with the vapor thereof.

surface to be coated a supply of evaporatable material in amount and distribution substantially constant per unit areal of the surface except adjacent to any end edge of the surface andA there materially greater in Vamount per unit area of the surface, and evaporating the mateirial to coat the surface with the vapor thereof.

l`2. The method of coating a. surface having an end edge with a coating which is uniform to the said edge and which method comprises` stepsl of positioning parallel to the surface and at an; angle to theend e'dge thereof an elongated body. of evaporable material substantially uniform vin amount per unit length except at any end thereof adjacent to an end edge 'of the surface and there of materially greater'amount per unit length than along the neighboring portionof the body, and evaporating the material to coat the '3. The method of coating a, surface of positioning .parallel to the surface .to be coated an elongated body of evaporable metal supported in a4 metallic helix, the said body being not parallel to the end edge of the surface and being of lsubstantially uniform amountv per unit length except at any endthereof adjacent to an end edge where the amount per unit length is materially greater than along the neighboring portion of the body, passing Athrough' the .helix and the body an electric current of sufficient amperage to fuse the successive short circuits of the body from coil to coil of the helix,

teen amperes for ten seconds, and then abruptly down to zero.

This completes the actual process of coating.

The vacuum' inside is broken by admitting air` through the tube 28, and the door or plug with its rods I1 and tray I0 areremoved. While the particular case and apparatus described relate to the coating of silver on quartz reducing the amperage of the current for a predetermined period to allow the fused and separated portions ofthe body to gather severally on the coils of the helixand raising the amperl age of the current to a sufficient amount and for a sufficient period of time to evaporate the metal of the divided body fromthe helix to the surface to coat the same.

'4. The method of coating a surfacev which comprises steps of positioning parallel to the surface to be0 coated an elongated body vof evaporable vmetal supported in a`metallic helix.`the"v said body being not parallel to the end edge of While having an end edge with `a coating which is 'uniformto the saidv edge and which method comprises steps amount mr unit le:` i: atany en d thereof adiacent to an `emi edge where the amount per unit le is materially greater than alons' the neighboring portion of the body, Passing through the helix and thebody an electric current of sumcieni; emperose to fuse the successive short circuits ci the body .from coil to coil oi the helix, reducing the amperalge oi' the cur- @$78,923 uniform- LOUIS GOLD. 

